The present invention relates generally to semiconductor processing systems and, more particularly, to a rapid thermal processing (RTP) system and method.
During the manufacture of semiconductor devices, certain processes require the temporary heating of the surface of semiconductor wafers in order to, for example, promote annealing processes or other reactions that may be desired. Conventionally, this heating process, which is here referred to as rapid thermal processing (RTP), is performed by heating the wafer with some form of external energy source such as, for example, a bank of tungsten-halogen lamps or a hot-wall furnace.
Recently, there has been renewed interest in very short heating cycles for processes such as annealing of ion-implantation damage for formation of ultra-shallow junctions. For example, a high temperature process may involve quickly heating a wafer to a peak temperature of approximately 1050° C. then immediately allowing the wafer to cool. Such a process is usually called a spike-anneal. In a spike-anneal process, it is desirable to heat the wafer to a high peak temperature in order to achieve good damage annealing and dopant activation, but the time spent at the high temperature should be as short as possible to avoid excessive dopant diffusion.
The technology trend in the last few years has been to increase the peak temperature of the spike-anneal while simultaneously decreasing the duration of time spent at the peak temperature. This modification is usually accomplished by increasing the heating ramp rate and the cooling rate, as well as by minimizing the switch-off time of the radiant heat source. These approaches help to minimize the peak-width of the spike-anneal, i.e., the time spent by the wafer above a given threshold temperature at which significant diffusion can rapidly occur. The peak-width is often characterized by considering the time spent above a threshold temperature, which is generally defined as 50° C. below the peak temperature of the spike-anneal heating cycle.
Additional methods to further reduce spike-anneal peak-widths are still being developed. For instance, one approach uses pulses of energy to heat the surface of the wafer. This approach takes advantage of the fact that the devices being processed lie within a relatively thin layer near the surface of the wafer and, therefore, there is no fundamental need to heat the entire wafer to the processing temperatures. Surface heating allows very rapid heating of the relatively small thermal mass of the surface region, as well as quick cooling because the heat in the surface region is dissipated into the bulk of the wafer by thermal conduction. Thermal conduction can be a much faster cooling mechanism in comparison to heat loss through radiation or convection from the wafer surface. Pulses of energy are typically applied from pulsed arc-lamps, such as xenon flashlamps, or by scanning an energy beam across the wafer surface. The energy beam may be, for example, a laser, an electron beam or any other type of energy source that may deliver a high power density at the wafer surface. These surface heating approaches are typically used to generate millisecond-duration heat pulses, and the process temperatures employed are higher than the ˜1050° C. temperatures used in conventional RTP. In some cases, annealing at temperatures as high as 1350° C. for approximately one millisecond has been shown to be useful. In the context of the present disclosure, a pulse is understood to refer to a short burst of energy such as, for example, a heat pulse with a duration on the order of milliseconds produced by a pulsed arc lamp. Another example is the use of a pulse of energy from a laser, which may be swept across a wafer surface in order to process a larger area. The laser used in such a context may be a pulsed or a continuous wave (cw) laser. In other words, a pulse is considered to be an energy burst of short duration, wherein the energy burst may be provided from one of a variety of sources.
Although the use of high power energy sources enables such a “millisecond annealing” approach, there are serious practical difficulties that remain to be resolved in this approach. For example, many of the practical pulsed energy sources rely on coupling of radiant energy to the wafer. Consequently, the radiant energy interacts strongly with the optical properties of the wafer. Since wafers are usually patterned with layers and devices that alter the optical properties across the surface of the wafer, the result is uneven power coupling to the wafer surface and potential temperature non-uniformity across the surface of the wafer. As the millisecond-annealing approach requires very high power densities to be applied to the wafer surface, even minor changes in optical coupling can result in large temperature gradients. These temperature gradients lead to non-uniform process results as well as large thermal stresses that can damage the devices on the wafer.
In particular, when a wafer is processed by exposure to energy from a radiant heat source or a beam of energy, non-uniformity may arise if different regions of the wafer absorb different amounts of energy from the incident beam of energy. Such pattern effects may arise, for instance, if the optical or physical properties of the surface vary across the wafer.
Turning now to the drawings, wherein like components are indicated by like reference numbers throughout the various figures where possible, attention is immediately directed to FIG. 1, which illustrates a partial cross sectional view of a typical semiconductor device, generally indicated by a reference numeral 10. It is noted that the figures are not drawn to scale for purposes of clarity. Device 10 includes a wafer 12 with a top surface 14 and a bottom surface 15. Device layers 16 and 18 are deposited on top surface 14. A trench 20 is also etched into top surface 14 of wafer 12 in order to facilitate separation between die, test areas (not shown) and edge exclusion areas (not shown), which are patterned in a different manner from the die. Other features such as edge exclusion regions and test areas may also be present on the wafer surface. Bottom surface 15 of wafer 12 is usually untreated. In general, a semiconductor device, such as device 10, may include thousands of such structures including a variety of patterned layers and trench-like features. Due to the presence of variations, such as represented by device layers 16 and 18 and trench 20, on top surface 14, aforedescribed temperature gradients across the wafer during heat application are common in semiconductor wafers because structures associated with different parts of the wafer vary in physical composition. As a result, optical properties may vary both within any one electronic circuit on the wafer and between different regions of the wafer. Severe variation in energy absorption may arise if different regions on the wafer have different reflectivity or scattering characteristics.
Unfortunately, in semiconductor devices, the thin film structures (such as device layers 16 and 18 of FIG. 1) present on the wafer often exhibit large differences in reflectivity, especially due to optical interference effects that occur in such thin film structures. The thin film stacks often include materials such as silicon dioxide (SiO2), silicon nitride (Si3N4), dielectrics, metals, metal silicides, metal nitrides or carbides, polycrystalline silicon (Si), amorphous Si, single-crystal Si, silicon germanium (SiGe) alloys or germanium (Ge).
Furthermore, semiconductor devices are structured into various line-shapes with three-dimensional topography. Variations in the line shapes and spacing between the lines also lead to variations in the power reflection and absorption. For example, optical energy may be diffracted by patterned features, and arrays of lines may act as grating structures that produce pronounced diffraction and scattering effects, all of which may further contribute to variations in energy absorption. The variations in energy absorption may consequently result in differences in the temperature rise produced or the degree to which a thermal process proceeds.
Another problem with the use of high power energy sources lies in the difficulty of providing adequate process control in terms of the repeatability and uniformity of the heating cycle. Since the process is typically accomplished in an extremely short time duration, it is difficult to apply, for example, conventional feedback control to regulate the heating process. It is also very difficult to apply accurate temperature measurement in the short timescale of the processing, especially when such power energy sources are applied to the wafer. When scanning energy sources are used, the problem is further complicated by the fact that the process occurs in a small region that is difficult to observe and control by conventional measurement and control techniques. Since the qualities of the energy sources may change or drift over time, there is no simple way to ensure that repeatable and uniform processing occurs at all points on all of the semiconductor wafers being processed.
Yet another problem arises from the challenging requirements of the high power energy sources themselves. Simple heat-flow calculations have shown that, for a one millisecond process time, it is typically necessary to deliver approximately 15 J/cm2 to the wafer surface. For example, for a 300 millimeter diameter wafer, the minimum total energy delivered to the wafer is approximately 11 kJ. If the energy is delivered to the wafer from, for instance, a bank of flashlamps, then energy is lost through absorption in the process chamber walls and other components, and, additionally, there is significant loss in the conversion of electrical energy to radiant emission from the lamps. As a result, the heating system typically needs to provide energy pulses of at least 50 kJ, which leads to rather large equipment. The alternative approach of scanning a focused energy beam across the wafer surface requires very expensive equipment. For example, if a high power laser is used, then expensive optics and wafer mounting fixtures are required. Moreover, the scanning approach limits the throughput in terms of the number of wafers that may be processed per hour.
Some types of energy sources used in thermal processing include electromagnetic energy sources, such as lamps, hot objects, lasers and microwave or millimeter wave sources, hot streams of gas, flames, particle beams (including streams of electrons or ions), plasmas, energetic atoms and radicals. Typical lamps may include tungsten halogen lamps or arc lamps. An arc lamp may include a flashlamp containing an inert gas such as xenon, argon, krypton, neon or mixtures of these gases. Lamps may be used in a direct current (DC) or pulsed mode (such as a flashlamp). In the pulsed mode, flashlamps usually produce pulses of energy with durations between 1 μs and 100 ms. One example of a suitable energy source is the high energy arc lamp manufactured by Mattson Technology, Canada.
The energy from lamps may be delivered to the wafer with the assistance of reflectors and other optical components. In many cases, it may be convenient to arrange the illumination such that all of the wafer may be exposed to the radiation in one pulse of energy. However, if so desired, the radiant energy may be delivered to various regions of the wafer sequentially while the wafer and/or energy source are moved with respect to each other between periods of illumination.
As will be seen hereinafter, the present invention provides a remarkable improvement over the prior art as discussed above by virtue of its ability to provide short heating cycles at high temperatures with increased performance while resolving the aforedescribed problems present in the current state of the art.